U.S. Senate Committee on Energy and Natural Resources
21 July 2005
Hearing on Climate Change Science and Economics
I thank Chairman Domenici, Ranking Member Bingaman, and the other Members of the Committee for the opportunity to speak with you today on the science of global climate change. My name is James W. Hurrell, Director of the Climate and Global Dynamics Division (CGD) at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. My personal research has centered on empirical and modeling studies and diagnostic analyses to better understand climate, climate variability and climate change. I have authored or co-authored more than 60 peer-reviewed scientific journal articles and book chapters, as well as dozens of other planning documents and workshop papers. I have given more than 65 invited talks worldwide, as well as many contributed presentations at national and international conferences on climate. I have also convened over one dozen national and international workshops, and I have served on several national and international science-planning efforts. Currently, I am extensively involved in the World Climate Research Programme (WCRP) on Climate Variability and Predictability (CLIVAR), and I serve as co-chair of Scientific Steering Committee of U.S. CLIVAR. I have also been involved in the assessment activities of the Intergovernmental Panel on Climate Change (IPCC) as a contributing author to chapters in both the third and fourth (in progress) assessment reports, and I have served on several National Research Council (NRC) panels. I am also a lead author on the U.S. Climate Change Science Program’s (CCSP) Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere.
Throughout this testimony I will refer to both the IPCC and the CCSP. Briefly, the IPCC is a body of scientists from around the world convened by the United Nations jointly under the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO). Its mandate is to provide policy makers with an objective assessment of the scientific and technical information available about climate change, its environmental and socio-economic impacts, and possible response options. The IPCC reports on the science of global climate change and the effects of human activities on climate in particular. The fourth major assessment is underway (the previous assessments were published in 1990, 1995 and 2001) and is due to be published in 2007. Each new IPCC report reviews all the published literature over the previous 5 years or so, and assesses the state of knowledge, while trying to reconcile disparate claims, resolve discrepancies and document uncertainties. For the 2001 Third Assessment Report (TAR), Working Group I (which deals with how the climate has changed and the possible causes) consisted of 123 lead authors, 516 contributors, 21 review editors, and over 700 reviewers. It is a very open process. The TAR concluded that climate is changing in ways that cannot be accounted for by natural variability and that “global warming” is happening.
The U.S. CCSP was established in 2002 to coordinate climate and global change research conducted in the United States. Building on and incorporating the U.S. Global Change Research Program of the previous decade, the program integrates federal research on climate and global change, as sponsored by 13 federal agencies and overseen by the Office of Science and Technology Policy, the Council on Environmental Quality, the National Economic Council and the Office of Management and Budget. A primary objective of the CCSP is to provide the best possible scientific information to support public discussion and government and private sector decision-making on key climate-related issues. To help meet this objective, the CCSP is producing a series of synthesis and assessment products that address its highest priority research, observation, and decision-support needs. Each of these products will be written by a team of authors selected on the basis of their past record of interest and accomplishment in the given topic. The Product on Temperature Trends in the Lower Atmosphere focuses on both understanding reported differences between independently produced data sets of temperature trends for the surface through the lower stratosphere and comparing these data sets to model simulations.
Observed Climate Change
a. Surface Temperature Improvements have been made to both land surface air temperature and sea surface temperature (SST) data during the five years since the TAR was published.The improvements relate to improved coverage, particularly over the Southern Hemisphere (SH) in the late 19th Century, and daily temperature data for an increasing number of land stations have also become available, allowing more detailed assessment of extremes, as well as potential urban influences on both large-scale temperature averages and microclimate.
The globe is warming. Claims to the contrary are not credible. Three different analyses of observations of surface temperature averaged across the globe show a linear warming trend of 0.6ºC ±0.2ºC since the beginning of the 20th Century. Rates of temperature rise are greater in recent decades: since 1979, global surface temperatures have increased more than 0.4ºC. Land regions have warmed the most (0.7ºC since 1979), with the greatest warming in the boreal winter and spring months over the Northern Hemisphere (NH) continents. A number of recent studies indicate that effects of urbanization and land-use change on the land-based temperature record are negligible as far as continental- and hemispheric-space averages are concerned, because the very real but local effects are accounted for. Recent warming is strongly evident at all latitudes over each of the ocean basins and, averaged over the globe, the SSTs have warmed 0.35ºC since 1979. The trends over the past 25 years have been fairly linear; however the global temperature changes over the entire instrumental record are best described by relatively steady temperatures from 1861-1920, a warming of about 0.3ºC to 1950, a cooling of about 0.1ºC until the mid-1970s, and a warming of about 0.55ºC since then. Thus, global surface temperatures today are about 0.75ºC warmer than at the beginning of the 20th Century.
The warmest year in the 145-year global instrumental record remains 1998, since the major 1997-98 El Niño enhanced it. The years 2002–2004 are the 2nd, 3rd and 4th warmest years in the series since 1861 and nine of the last 10 years (1995 to 2004) – the exception being 1996 – are among the ten warmest years in the instrumental record. Based on reconstructions of temperature from proxy data, like tree rings and ice cores, several studies have also concluded that NH surface temperatures are warmer now than at any time in at least the last 1,000 years.
b. Consistency with other observed changes The warming described above is consistent with a body of other observations that gives a consistent picture of a warming world. For example, there has been a widespread reduction in the number of frost days in middle latitude regions, principally due to an earlier last day of frost in spring rather than a later start to the frost season in autumn. There has been an increase in the number of warm extremes and a reduction in the number of daily cold extremes, especially at night. The amount of water vapor in the atmosphere has increased over the global oceans by 1.2 ± 0.3% from 1988 to 2004, consistent in patterns and amount with changes in SST and a fairly constant relative humidity. Widespread increases in surface water vapor are also found. Ocean temperatures have warmed at depth as well, and global sea levels have risen 15-20 centimeters over the 20th Century: as the oceans warm, seawater expands and sea level rises.
There has been a nearly worldwide reduction in mountain glacier mass and extent. Snow cover has decreased in many NH regions, particularly in the spring season and this is consistent with greater increases in spring than autumn surface temperatures in middle latitude regions. Sea-ice extents have decreased in the Arctic, particularly in the spring and summer seasons, and patterns of the changes are consistent with regions showing a temperature increase. The Arctic (north of 65ºN) average annual temperature has increased since the 1960s and is now warmer (at the decade timescale) than conditions experienced during the 1920–1945 period (where much of the earlier global warming was centered). In the Antarctic, there are regional patterns of warming and cooling related to changes in the atmospheric circulation. The warming of the Peninsula region since the early 1950s is one the largest and the most consistent warming signals observed anywhere in the world. Large reductions in sea-ice have occurred to the west in the Bellingshausen Sea, and on the eastern side of Peninsula, large reductions in the size of Larsen Ice shelf have occurred.
c. Temperature of the Upper Air
Radiosonde releases provide the longest record of upper-air measurements, and these data show similar warming rates to the surface temperature record since 1958. Unfortunately, however, vast regions of the oceans and portions of the landmasses (especially in the Tropics) are not monitored so that there is always a component of the global or hemispheric mean temperature that is missing. Moreover, like all measurement systems, radiosonde records of temperature have inherent uncertainties associated with the instruments employed and with changes in instrumentation and observing practices, among other factors. Fundamentally, these uncertainties arise because the primary purpose of radiosondes is to help forecast the weather, not monitor climate variability and change. Therefore, all climate data sets require careful examination for instrument biases and reliability (quality control) and to remove changes that might have arisen for non-climatic reasons (a process called “homogenization”.) It is difficult to remove all non-climatic effects, and ideally multiple data sets should be produced independently to see how sensitive results are to homogenization choices. This has been the case for the surface record, but unfortunately much less so for the radiosonde record (although efforts are increasing.)
For this reason, much attention has been paid to satellite estimates of upper-air temperatures, in particular because they provide true global coverage. Of special interest have been estimates of tropospheric and stratospheric temperatures over thick atmospheric layers obtained from microwave sounding units (MSU) onboard NOAA polar-orbiting satellites since 1979. Initial analyses of the MSU data by scientists at the University of Alabama, Huntsville (UAH) indicated that temperatures in the troposphere showed little or no warming, in stark contrast with surface air measurements. Climate change skeptics have used this result to raise questions about both the reliability of the surface record and the cause of the surface warming, since human influences thought to be important are expected to increase temperatures both at the surface and in the troposphere. They also have used the satellite record to caste doubt on the utility of climate models, which simulate both surface and tropospheric warming in over recent decades.
In an attempt to resolve these issues, the NRC in 2000 studied the problem and concluded that “the warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the 20th Century. The disparity between surface and upper air trends in no way invalidates the conclusion that surface temperature has been rising.” The NRC further found that corrections in the MSU processing algorithms brought the satellite data record into slightly closer alignment with surface temperature trends, but substantial discrepancies remained. As further noted by the TAR, some, but not all, of these remaining discrepancies could be attributed to the fact that the surface and the troposphere respond differently to climate forcings, so that trends over a decade or two should not necessarily be expected to agree.
Since the IPCC and NRC assessments, new data sets and modeling simulations have become available which are helping to resolve this apparent dilemma. The CCSP Assessment Product on Temperature Trends in the Lower Atmosphere is assessing these new data, and the preliminary report (which has been reviewed by the NRC) finds that the surface and upper-air records of temperature change can now, in fact, be reconciled. Moreover, the overall pattern of observed temperature change in the vertical is consistent with that simulated by today’s climate models.
Several developments since the TAR are especially notable:
A second, independent record of MSU temperatures has become available from scientists at the Remote Sensing Systems (RSS) Laboratory. Although both the UAH and RSS groups start from the same raw radiance data, they apply different construction methods of merging the MSU data from one satellite to the next. The result is that, while both data sets indicate the middle troposphere has warmed since 1979, the RSS estimate is ~0.1ºC decade-1 warmer than the UAH estimate. Moreover, the RSS trend is not statistically different from the observed surface warming since 1979. The difference in tropospheric temperature trends between these two products highlights the issue of temporal homogeneity in the satellite data.
Both UAH and RSS MSU products support the conclusion that the stratosphere has undergone strong cooling since 1979, due to observed stratospheric ozone depletion.
Because about 15% of the MSU signal for middle tropospheric temperature actually comes from the lower stratosphere, the real warming of the middle troposphere is greater than that indicated by the MSU data sets. This has been confirmed by new analyses that explicitly remove the stratospheric influence, which is about –0.08ºC decade-1 on middle tropospheric MSU temperature trends since 1979.
By differencing MSU measurements made at different slant angles, both the UAH and the RSS groups have produced updated data records weighted more toward the lower troposphere. The RSS product exhibits a warming trend that is 0.2ºC decade-1 larger than that from UAH. In part, this discrepancy is because adjustments for diurnal cycle corrections required from satellite drift had the wrong sign in the UAH record. As a result, a new UAH record is being prepared, and the current version is regarded as obsolete.
The various new data sets of upper-air temperature are very important because their differences highlight differences in construction methodologies. It therefore becomes possible to estimate the uncertainty in satellite-derived temperature trends that arises from different methods.
d. Extremes For any change in mean climate, there is likely to be an amplified change in extremes. The wide range of natural variability associated with day-to-day weather means that we are unlikely to notice most small climate changes except for changes in the occurrence of extremes. Extreme events, such as heat waves, floods and droughts, are exceedingly important to both natural systems and human systems and infrastructure. We are adapted to a range of natural weather variations, but it is the extremes of weather and climate that exceed tolerances.
In several regions of the world indications of a change in various types of extreme weather and climate events have been found. So far, the most prominent indication of a change in extremes is the evidence of increases in moderate to heavy precipitation events over the middle latitudes in the last 50 years, even for regions where annual precipitation totals are decreasing. Further indications of a robust change include the observed trend to fewer frost days associated with the average warming in most middle latitude regions. Results for temperature-related daily extremes are also relatively coherent for some measures. Many regions show increased numbers of warm days/nights (and lengthening of heat waves) and even more reductions in the number of cold days/nights, but changes are not ubiquitous.
Trends in tropical storm frequency and intensity are masked by large natural variability on multiple timescales. Increases may be occurring in recent years, but apart from the North Atlantic basin, most measures only begin in the 1950s or 1960s and have likely missed some events in the early decades. Numbers of hurricanes in the North Atlantic have been above normal in 8 of the last 10 years, but levels were about as high in the 1950s and 1960s. This pattern continues this summer, with a very active hurricane season already evident and SSTs at record high levels.
Modeling and Attribution of Climate Change
a. Improved simulations of past climate
The best climate models encapsulate the current understanding of the physical processes involved in the climate system, the interactions, and the performance of the system as a whole. They have been extensively tested and evaluated using observations. They are exceedingly useful tools for carrying out numerical climate experiments, but they are not perfect, and some models are better than others. Uncertainties arise from shortcomings in our understanding of climate processes operating in the atmosphere, ocean, land and cryosphere, and how to best represent those processes in models. Yet, in spite of these uncertainties, today’s best climate models are now able to reproduce the climate of the past century, and simulations of the evolution of global surface temperature over the past millennium are consistent with paleoclimate reconstructions.
As a result, climate modelers are able to test the role of various forcings in producing the observed changes in global temperature temperatures. Forcings imposed on the climate system can be natural in origin, such as changes in solar luminosity or volcanic eruptions, the latter adding considerable amounts of aerosol to the upper atmosphere for up to two years. Human activities also increase aerosol concentrations in the atmosphere, mainly through the injection of sulfur dioxide from power stations and through biomass burning. A direct effect of sulfate aerosols is the reflection of a fraction of solar radiation back to space, which tends to cool the Earth’s surface. Other aerosols (like soot) directly absorb solar radiation leading to local heating of the atmosphere, and some absorb and emit infrared radiation. A further influence of aerosols is that many act as nuclei on which cloud droplets condense, affecting the number and size of droplets in a cloud and hence altering the reflection and the absorption of solar radiation by the cloud. The precise nature of aerosol/cloud interactions and how they interact with the water cycle remains a major uncertainty in our understanding of climate processes. Because man-made aerosols are mostly introduced near the Earth’s surface, they can be washed out of the atmosphere by rain. They therefore typically remain in the atmosphere for only a few days, and they tend to be concentrated near their sources such as industrial regions. Therefore, they affect climate with a very strong regional pattern and usually produce cooling.
In contrast, greenhouse gases such as carbon dioxide and methane are not washed out, so they have lifetimes of decades or longer. As a result, they build up in amounts over time, as has been observed. Greenhouse gas concentrations in the atmosphere are now higher than at any time in at least the last 750,000 years. It took at least 10,000 years from the end of the last ice age for levels of carbon dioxide to increase 100 parts per million by volume (ppmv) to 280 ppmv, but that same increase has occurred over only the past 150 years to current values of over 370 ppmv. About half of that increase has occurred over the last 35 years, owing mainly to combustion of fossil fuels and deforestation. In the absence of controls, future projections are that the rate of increase in carbon dioxide amount may accelerate, and concentrations could double from pre-industrial values within the next 50 to 100 years.
Climate model simulations that account for such changes in forcings have now reliably shown that global surface warming of recent decades is a response to the increased concentrations of greenhouse gases and sulfate aerosols in the atmosphere. When the models are run without these forcing changes, they fail to capture the almost linear increase in global surface temperatures since the mid-1970s. But when the anthropogenic forcings are included, the models simulate the observed temperature record with impressive fidelity. These same model experiments also reveal that changes in solar luminosity account for much of the warming in the first half of the 20th Century. Such results increase our confidence in the observational record and our understanding of how temperature has changed. They also mean that the time histories of the important forcings are reasonably known, and that the processes being simulated models are adequate enough to make the models very valuable tools.
b. Commitment to further climate change
The ability of climate models to simulate the past climate record gives us increased confidence in their ability to simulate the future. Moreover, the attribution of the recent climate change to increased concentrations of greenhouse gases in the atmosphere has direct implications for the future. Because of the long lifetime of carbon dioxide and the slow equilibration of the oceans, there is a substantial future commitment to further global climate change even in the absence of further emissions of greenhouse gases into the atmosphere. Several modeling groups have performed “commitment” runs in order to examine the climate response even if the concentrations of greenhouse gases in the atmosphere had been stabilized in the year 2000. The exact results depend upon the model, but they all show a further global warming of about another 0.5ºC, and additional and significant sea level rises caused by thermal expansion of the oceans by the end of the 21st Century. Further glacial melt is also likely.
The climate modeling groups contributing to the Fourth IPCC Assessment Report have produced the most extensive internationally coordinated climate change experiment ever performed (21 global coupled models from 14 countries). This has allowed better quantification of multi-model responses to three scenarios of 21st century climate corresponding to low (550 ppmv), medium (690 ppmv) and high (820 ppmv) increases of carbon dioxide concentrations by the year 2100. In spite of differences among models and the uncertainties that exist, the models produce some consistent results:
Over the next decade or two, all models produce similar warming trends in global surface temperatures, regardless of the scenario.
Nearly half of the early 21st Century climate change arises from warming we are already committed to. By mid-century, the choice of scenario becomes more important for the magnitude of warming, and by the end of the 21st Century there are clear consequences for which scenario is followed.
The pattern of warming in the atmosphere, with a maximum in the upper tropical troposphere and cooling in the stratosphere, becomes established early in this century.
Geographical patterns of warming show greatest temperature increases at high northern latitudes and over land, with less warming over the southern oceans and North Atlantic. In spite of a slowdown of the meridional overturning circulation and changes in the Gulf Stream in the ocean across models, there is still warming over the North Atlantic and Europe due to the overwhelming effects of the increased concentrations of greenhouse gases.
Precipitation generally increases in the summer monsoons and over the tropical Pacific in particular, with general decreases in the subtropics and some middle latitude areas, and increases at high latitudes.
c. Increasing complexity of models
As our knowledge of the different components of the climate system and their interactions increases, so does the complexity of climate models. Historical changes in land use and changes in the distribution of continental water due to dams and irrigation, for instance, need to be considered. Future projected land cover changes due to human land uses are also likely to significantly affect climate, and these effects are only now being included in climate models.
One of the major advances in climate modeling in recent years has been the introduction of coupled climate-carbon models. Climate change is expected to influence the capacities of the land and oceans to act as repositories for anthropogenic carbon dioxide, and hence provide a feedback to climate change. These models now allow us to assess the nature of this feedback. Results show that carbon sink strengths are inversely related to the rate of fossil fuel emissions, so that carbon storage capacities of the land and oceans decrease and climate warming accelerates with faster carbon dioxide emissions. Furthermore, there is a positive feedback between the carbon and climate systems, so that further warming acts to increase the airborne fraction of anthropogenic carbon dioxide and amplify the climate change itself.
In summary, the scientific understanding of climate change is now sufficiently clear to show that climate change from global warming is already upon us. Uncertainties remain, especially regarding how climate will change at regional and local scales. But the climate is changing and the uncertainties make the need for action all the more imperative. At the same time, it should be recognized that mitigation actions taken now mainly have benefits 50 years and beyond now. This also means that we will have to adapt to climate change by planning for it and making better predictions of likely outcomes on several time horizons. My personal view it that it is vital that all nations identify cost-effective steps that they can take now, to contribute to substantial and long-term reductions in net global greenhouse gas emissions. Action taken now to reduce significantly the build-up of greenhouse gases in the atmosphere will lessen the magnitude and rate of climate change. While some changes arising from global warming are benign or even beneficial, the rate of change as projected exceeds anything seen in nature in the past 10,000 years. It is apt to be disruptive in many ways. Hence it is also vital to plan to cope with the changes, such as enhanced droughts, heat waves and wild fires, and stronger downpours and risk of flooding. Managing water resources will be major challenge in the future.
Again, I appreciate the opportunity to address the Committee concerning the science of global climate change – a topic that is of the utmost importance for the future of our planet.
* Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author and do not necessarily reflect those of the National Science Foundation.
** The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation.